Medical implantable leads of the above kind are well known in the art and can be employed whenever it is desirable to monitor or control an organ inside a human or animal body by receiving and/or transmitting electric signals from and to the tissue of the organ. One such common application is monitoring and controlling of a heart by means of a pacemaker or an ICD (Implantable Cardioverter Defibrillator), wherein the pacemaker or the ICD is connected to a proximal end of the lead whereas a distal end of the lead and in particular at least one electrode positioned at the distal end or distal portion of the lead is in contact with tissue of the heart. However, it is to be understood that it could be conceivable to electrically monitor and/or control also other organs inside a human or animal body.
To accomplish electrical monitoring and/or controlling of an organ inside a body, it is common practice to insert the lead into the body through some kind of narrow body canals or vessels. In the case of monitoring and/or controlling of a heart by means of a pacemaker or an ICD, this vessel typically is a vein through which the distal end of the lead can be brought into a ventricle or an atrium of the heart or be placed inside a coronary vein surrounding the heart. The latter is frequently applied when implanting a left ventricular lead (LV-lead) to a heart for the purpose of not causing any blood cloths inside the left ventricle. A blood cloth there could be potentially hazardous for the patient since the left ventricle is in direct connection with the brain and any coagulated blood in the blood flow could cause an obstruction in the small capillaries of the brain. Therefore an LV-lead is usually positioned in one of the coronary veins surrounding the left ventricle of the heart.
Since a lead of this kind is to be inserted into the body through narrow body vessels, such as veins, an important feature for such a lead is that it should have a small cross sectional dimension to facilitate and even at all allow insertion through the desired vessel. To accomplish this it is common practice to form for example an LV-lead of a flexible tubing, of e.g. silicone, and to arrange one or more cables, i.e. electrically conducting wires surrounded by an electrically insulating layer, inside the tubing which connects the one or more electrodes at the distal portion of the lead with electric contact surfaces at a connector in the proximal end. In order to make the lead sufficiently rigid to be able to insert the lead into a narrow vein, provisions are taken to allow insertion of a stiffening stylet into an inner bore of the tubing, wherein the stylet is inserted into the tubing during introduction of the lead into the vein and is subsequently withdrawn. The outer cross sectional dimension of a lead designed in this way can be kept advantageously small.
However, it has become more and more common throughout the world to use magnetic resonance imaging (MRI) for diagnosing and examining bodies for various diseases or injuries. The use of MRI is increasing, both as the MRI equipment becomes less expensive, less complicated and smaller but also as the number of applications increase. For a person or animal having a lead relating to the present art implanted into the body, it is of great benefit that the lead is MRI-compatible. MRI-compatible as used herein implies that any heating of electrodes in connection with the distal end of the implantable medical lead caused by a current induced by RF fields of the MRI system is at an acceptable level to thereby not cause or at least reduce the risk of causing significant injuries to surrounding tissue in the subject body or damages to internal lead parts. By a lead which is not MRI-compatible, the magnetic field will induce current in the lead which can lead to an incorrect stimulation of an organ, e.g. a heart, or thermal damage to the tissue closest to the electrodes. It can also render the MRI procedure unusable.
Over the years it has been suggested different ways to render a medical implantable lead MRI-compatible. One way is to arrange the one or more conductors, which electrically connects the one or more electrodes at the distal end portion of the lead with the connector in the proximal end, in form of one or more coils of one or more electrically conducting wires, each having an electrically insulating layer. A coil formed in this way will act as an inductance coil which will prevent or reduce the induction of high frequent alternating current into the coil. A lead arranged in this way is disclosed in e.g. WO 2010064962 to the present assignee.
One disadvantage with a lead having an electric conductor in form of a coil is however that it will in most cases have a larger cross sectional dimension in comparison to a corresponding lead being provided with a conductor in form of one or more cables. Moreover, the inductance of the coil, and accordingly also the MRI-compatibility, will increase with an increased diameter of the coil so it is advantageous to try to keep the cross sectional dimension as large as possible. However, even a small increase of the cross sectional dimension can cause problems when it comes to insertion of the lead through narrow veins or other narrow body vessels. The smaller the lead diameter is, the more veins it can be placed in and also the further it can be inserted into a particular vein. For an LV-lead the placement is very important for the effectiveness of the lead function, so being able to position the lead at the most desirable position is very important.
From U.S. pat. No. 5,755,766 it is known a medical implantable lead having a conductor in form of a coil and which has a reduced lead body diameter in a most distal portion. The reduced lead body diameter is achieved by reducing the diameter of the coil in the most distal portion. However, a coil formed in this way will be comparatively expensive and complicated to manufacture.